SMT007 Magazine

SMT-Oct2017

Contents of this Issue

Navigation

Page 11 of 97

12 SMT Magazine • October 2017
Following the prelude for this series (SMT
Magazine, August 2017), we now sequentially
address the topics as outlined therein.
Elemental Properties
Bismuth (atomic number 83 and atomic
weight 208) is classified as a metal. It normal-
ly appears as grayish white with reddish tinge
and can be grown into colorful iridescent crys-
tals. It is soft, but brittle. It has rhombohedral
crystal structure in contrast to a cubic structure
of lead and tetrahedral of tin. It melts at 271°C
(520°F) and boils at 1,560°C (2,840°F). In
comparison with tin
and lead, its density
(9.80g/cm
3
) is high-
er than Sn (7.31g/
cm
3
) and lower than
Pb (11.34g/cm
3
).
With respect to
conductivity, the
electrical conductivi-
ty of bismuth is mea-
sured at 0.8 (10
4
Ohm
–1
cm
-1
) versus tin at
9.1 and lead at 4.8,
and the thermal con-
ductivity is around 8
(w/m-k, 300°K) com-
pared to tin's 66 and
lead's 35. Among
metals, bismuth's
electrical and ther-
mal conductivity are the lowest. The coefficient
of thermal expansion (CTE) is also lower than
that of tin or lead, at 13.4 x 10
-6
/C (Sn=22.0,
Pb=28.9). The lower CTE can be leveraged as a
useful property to design a proper CTE of solder
materials.
Its lower surface tension (378mN/m,
270°C) than tin (574mN/m, 232°C) and lead
(465mN/m, 327°C) is also a useful proper-
ty, contributing
to the improve-
ment in wet-
ting ability. This
improvement is
expected to be
obser
ved in bis-
muth-containing
solders with
oth-
er conditions being
equal. This is con-
sidered a significant
advantage over
oth-
er elements when the
specific performance
requirement is need-
ed.
Two other unique
properties of Bi are that
it has the greatest Hall
effect of any metals (i.e.,
its resistance increases in
a magnetic field), and that
Bi expands upon solid-
by Dr. Jennie S. Hwang
H-TECHNOLOGIES GROUP
The Role of Bismuth (Bi) in Electronics,
Part 1
SMT PROSPECTS & PERSPECTIVES